A conventional axial flow gas turbine engine includes an array of turbine blades which extend through a flow path for hot gases, or working fluid, exiting a combustion section. As a result of the engagement with the working fluid flowing through the flowpath, the array of blades rotate about a longitudinal axis of the gas turbine engine. Efficient operation of the turbine requires minimizing the amount of working fluid which bypasses the turbine blades as the working fluid flows through the turbine. One method of accomplishing this is to provide an annular shroud which extends about the array of turbine blades in close radial proximity to the radially outward tips of the turbine blades. Modern gas turbine engines typically use shrouds comprised of a plurality of segments which are circumferentially aligned to form the annular shroud.
Each shroud segment includes a substrate having means to retain the segment to the support structure of the turbine section and a flow surface facing the blade tips and exposed to the working fluid. In order to minimize the gaps between the flow surface and the blade tips, the flow surface may include an abradable coating. The abradable coating permits the blade tips to make contact with the segments during operation without damaging the blades. In effect, the blades and segments are tolerant of thermal growth during operation without significantly degrading efficiency.
Since the shroud segment is in contact with the hot gases of the working fluid, means to maintain the shroud segment within acceptable temperature limits is required. One means of cooling the segments is to flow some of the compressor fluid directly to the segments. This cooling fluid impinges upon the radially outer surface of the shroud segment and removes some heat from the segment. Another technique to minimize the temperature of the segment is to form the abradable layer from a ceramic material. The ceramic abradable coating provides insulation between the hot working fluid and the substrate. Further techniques include film cooling the abradable layer.
The means of retention is typically a hook type structure, either a plurality of individual hooks or a circumferentially extending rail, disposed on the upstream and downstream ends of the segment. The retention means engages with the support structure to radially retain the segment. The support structure may also include a pin which engages with an accommodating cut-out in the segment to position the segment laterally.
Sealing mechanisms are used to prevent cooling fluid from bypassing the segment and flowing between adjacent segments or between the segments and the support structure. Conventional sealing mechanisms for segments include feather seals and `W` seals. Feather seals extend laterally between adjacent segments to seal this opening. `W` seals are disposed between the segments and the support structure to seal this opening. The `W` seals usually require a laterally extending sealing surface on the segment to engage the `W` seal. Due the presence of this sealing surface along the axial edges, the hooks and rails extend further outward from the substrate and present a larger profile.
Shroud segments, since they are exposed to extreme temperatures and abrasive contact from the rotating blades, are replaced frequently. A large temperature gradient may exist between the radially outer surfaces of the substrate, exposed to cooling fluid, and the flow surface, which is exposed to the working fluid. The temperature gradient and the thermal expansion that results from it cause the segment to distort. This distortion may increase the destructive contact between the segment and the blade. Another problem occurs, however, if the segment is stiffened, such as by having an extending rail. Even if spaced hooks are used, which are inherently more flexible than an extending rail, may not permit sufficient flexibility especially if the presence of a `W` seal requires large profile hooks be used. In this case, compressive stresses may be induced in the substrate and the ceramic abradable layer as a result of the segment not being permitted to distort enough to accommodate the thermal deflection. This may lead to cracking of the substrate, the abradable layer, or both. Another concern is the size and weight of the segments.
One possible solution is to remove the `W` seal and have short, individual hooks as the retaining means. This would provide insufficient sealing and require additional cooling fluid be drawn from the compressor. Another solution is to have a continuous rail which fits snugly within the support structure to provide the needed sealing. This configuration, however, would not accommodate thermal growth of the segment and would result in thermal stress related damage to the segment or support structure. Having a loose fitting rail and accepting some cooling fluid loss would accommodate some thermal expansion, but would introduce a variation in the radial positioning of the segment. This variation would produce larger radial gaps between the blade and the shroud and result in less efficient engagement between the blades and the working fluid.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop thin, flexible shroud segments which distort under operating conditions without degrading performance of the gas turbine engine.